Cryptology ePrint Archive: Report 2002/079

On the efficiency of the Clock Control Guessing Attack

Erik Zenner

Abstract: Many bitstream generators are based on linear feedback shift registers. A widespread technique for the cryptanalysis of those generators is the linear consistency test (LCT). In this paper, we consider an application of the LCT in cryptanalysis of clock-controlled bitstream generators, called \textsl{clock control guessing}. We give a general and very simple method for estimating the efficiency of clock control guessing, yielding an upper bound on the effective key length of a whole group of bitstream generators. Finally, we apply the technique against a number of clock-controlled generators, such as the A5/1, alternating step generator, step1-step2 generator, cascade generator, and others.